1. Field of the Invention
The invention relates to a method and a device for the continuous production of glass and glass ceramic products from a glass melt.
2. Description of Related Art
Glass products, such as, in particular, high-purity glasses and glass ceramics, are generally produced in melt vessels made from noble metals, such as platinum or platinum alloys, as well as from silica glass. However, these have known drawbacks, such as, for example, a yellowing due to ionic platinum entrained in the glass melt and/or scattering effects at entrained platinum particles as well as streaks and other inhomogeneities due to dissolution of the silica glass crucible material in the glass melt.
In addition, glass melts for high-purity glasses and glass ceramics are often quite aggressive toward the crucible materials used in each case. As a result, wear of the equipment and a premature end of the production occur.
Known from DE 102 44 807 A1 is a remedy for these drawbacks through the use of a so-called skull melting unit, which comprises a multi-turn coil constructed from water-cooled copper pipes and a skull crucible constituted of pipes made of metal (Cu, Al, Ni—Cr—Fe alloy, or possibly Pt) and having a palisade-like arrangement parallel to the coil axis, which are referred to also as “cold fingers.” The pipes of the skull crucible must have a minimum spacing in order to enable the applied high-frequency electric field to penetrate into the fluid glass present in the skull crucible and to heat it further by direct in-coupling with the creation of eddy currents. A crust of solidified/crystallized intrinsic material forms between the cooled metal crucible and the hot glass. This has the function of protecting the metallic crucible against corrosive glass attack and of protecting the glass against the entrainement of impurities from the metal and forms a leakage barrier and effects a reduction of the heat losses from the glass to the cooling medium.
These functions are fulfilled by the cited melting method. Furthermore, it is possible to produce glass products having good quality. However, the melting method has the drawbacks presented below.
The high operating voltages of greater than 1000 V result repeatedly in flashovers, mostly between the coil and the crucible and especially in dusty surroundings. This can result in long-lasting interruptions in operation and thus lead to high production costs.
The high voltages pose a potential source of danger to the persons operating the unit.
As a result, idle powers of 10 to 20% of the total power are created, in particular due to the voltage drop at the crucible.
In addition, in the metallic pipes having a palisade-like arrangement, which form the side walls of the crucible, as well as in the metallic bottom, energy is absorbed or reflected and is no longer available for the melting process. In order to enable at all a heating of the melt by means of the applied high-frequency electric field, it is necessary to ensure an energy input that is as efficient as possible. Losses in the metallic materials that belong to the melting unit must be minimized to the greatest degree possible. Opposed to the use of ceramics in the melting unit, however, is the high corrosiveness toward ceramic materials that many glass and glass ceramic melts display. If ceramics made of refractory components are used for the melting unit, therefore, there is no adequate leakage protection. In addition, the dissolution products of the ceramic linings result in streaks, bubbles, discolorations, and other flaws in the glass, which can substantially impair the quality of the product.
Another problem posed by melt crucibles in general and thus also by skull crucibles is changing the glass composition in a continuously operating unit. A conventional approach consists in simply changing the composition of the batch that is introduced during the course of the melting. In doing so, there exists the problem that the glass produced after the change in the batch composition only gradually assumes the desired composition. In this case, at least insofar as the difference in the composition is appreciable for the desired glass properties, substantial quantities of produced glass having a mixed composition cannot be used. In addition, the unproductive period of time until the attainment of the desired glass composition leads to increased production costs.
If the crucible is cooled, on the other hand, the solidified glass residues must be removed. However, these adhere to the surfaces of the crucible that are in contact with the melt. If the glass is knocked off, the crucible may be damaged.
Therefore, the problem of the invention is to simplify changing between the two kinds of glass.